1. Field of the Art
This invention relates to a method and an apparatus for producing vitreous optical elements by infection molding, i.e., by introducing heated molten -glass into a mold cavity having transfer surfaces to be copied onto glass moldings.
2. Prior Art
In the manufacture of precision optics like optical glass lenses, it has long been the conventional practice to resort to a grinding and polishing process. However, the conventional grinding and polishing processes have a number of drawbacks such as low production efficiency, difficulty of producing aspheric lenses etc. As a substitute for the conventional grinding and polishing process, it has also been known in the art to produce optical elements by compression molding using preformed glass of a predetermined shape, i.e., by heating and softening a preform of glass and compressing same to shape within a die. Further, developments have been made for injection molding processes in regard to the production of optical elements of the class as mentioned above. Of these molding processes, the compression molding process requires to prepare a large number of preforms beforehand, while the injection molding process which is free of problems of this sort is superior in terms of production efficiency.
In an injection molding process, glass material is heated to lower its viscosity to a working point, and resulting molten glass of low viscosity is injected into a mold under a predetermined pressure to mold a vitreous optical element having contours defined by a mold cavity. In addition to the mold cavity with transfer surfaces, a mold assembly unit for use in injection molding is normally provided with a sprue in communication with the cavity for introduction of molten glass into the latter. At one end, the sprue is opened into the mold cavity, and, at the other end, opened on the outer periphery of a shell block of the mold assembly. Heated molten glass, maintained in a fluidized state by reduction in viscosity, is injected into the sprue under a predetermined pressure to fill in the mold cavity completely.
Subsequent to injection of molten glass into the mold cavity, the whole mold assembly unit is cooled down to let glass solidify to the shape of the mold cavity, copying the contours defined by mold cavity walls. Accordingly, a molded optical element of predetermined shape or contours is obtained upon ejecting the molded product from the mold and cutting off surplus glass from the sprue.
The temperature of glass working point, at which glass has suitable viscosity in injection molding, is normally higher than 600.degree. C. depending upon the properties of the glass material concerned, and at least higher than 550.degree. C. even in a case where glass material of particular properties is selected. Namely, prior to introduction into a mold, glass material is heated into a molten state having a temperature in the vicinity of or higher than the glass working point, and then cooled down for solidification of glass which initially takes place at a transformation temperature in the range of from 450.degree. C. to 400.degree. C. Glass stays in a fluidized state until the transformation temperature, and undergoes thermal contractions on further temperature drops. The thermal contractions of this sort gives rise to the problems of surface sink or shrink marks which occur to the surfaces of solidified glass within the mold as a result of thermal contraction, making it difficult to obtain products of high precision quality surface contour.